The inventor of the present invention filed applications about an artificial intervertebral disc showing biomimetic mobility of intervertebral disc while binding tightly to endplates of the vertebral bodies well, in the case of inserting into a space between the vertebral bodies and receiving a biological load. One of them is an artificial intervertebral disc where a bioactive and bioresorbable solid pin(s) with high bioactivity and bio-absorbability is vertically penetrated through a structured fabric, which is a cubic multi-axial three-dimensional woven or knitted fabric of organic fibers, or, a complex structured fabric of them, and where both ends of the pin(s) are protruded from both upper and lower surfaces of the structured fabric (Patent Literature 1); and the other is an artificial intervertebral disc where the pin(s) is vertically penetrated through the inside of the structured fabric, and where the structured fabric is compressed from top and bottom sides, it is designed such that both ends of the pin are protruded from both the upper and lower surfaces of the structured fabric (Patent Literature 2). These are the stand-alone type of artificial intervertebral discs where both ends of a pin(s) protruding from both the upper and lower surfaces of a structured fabric are fitted into hole(s), which is created in endplate surfaces of the upper and lower vertebral bodies and fixed.
However, as results from insertion and mobility tests into human cadavers, and implanting tests into living baboon and other in vitro mobility tests, the following disadvantages has been recognized. In other words, when the structured fabric composed of organic fibers in as the artificial intervertebral disc is penetrated through or the inside is intruded through so as to fix the structured fabric with a pin(s) protruding from the upper and lower surfaces, it became ascertained that problems below remain. Namely, the problem described below still remain in the case that the structured fabric composed of organic fibers as the artificial intervertebral disc is fixed by the rigid pin(s), which penetrated through the fabric body or just intruded the inside protruding from top and bottom surfaces.
One problem as described below is such that insertion and fixation of the artificial intervertebral disc into/to the disc space between the vertebral bodies are difficult, and there is risk to damage the vertebral bodies or adjacent vertebral bodies due to forcible insertion.
When the artificial intervertebral disc is inserted and fixed into/to a vertebral space (disc space), which is reduced by curettage of a damaged biological intervertebral disc, first, punched holes are created in endplate surfaces contacting to the upper and lower vertebral bodies using an endplate puncher, and both ends of pins protruding from both the upper and lower surfaces of the structured fabric as an artificial intervertebral disc are fitted into these holes and a condition is reduced for inserting and fixing the artificial intervertebral disc at right positions. In order to insert the artificial intervertebral disc to the positions where the pins are set up rightly at the hole positions, because the space between the vertebral bodies has to be extended to the height including both protruding ends of the pins, it is natural that adjacent intervertebral discs located at the next superior and inferior positions should be compressed due to pressure of vertebral bodies shifted by the expanding force. In order to certainly fix the artificial intervertebral disc, the necessary length of one end of the pin protruding from the surface of the structured fabric in the artificial intervertebral disc (protruding distance) is empirically 1.0 mm to 3.0 mm, and is preferably 1.0 mm to 2.0 mm at least.
When the artificial intervertebral disc is inserted, the structured fabrics as the artificial intervertebral disc is interposed with two blades at the end portion of a catalyst as jigs. On this occasion, a dent channel with the same depth as the protruding length of the pin or a cutting channel penetrating from the top through the bottom of the blade is established in this blade to insert the artificial intervertebral disc into an intervertebral space under the ends of the protruding pin are fitted into the channels as to be adopted. However, the dent channel with the same depth as the protruding distance of the pin ends is established in the blade, which is thicker than the protruding distance of the pin end, because the thickness of the blade is greater than the protruding distance of the pin end, the intervertebral space has to be further extended to allow for the thickness of this blade. With this result, adjacent undamaged intervertebral discs located in the next disc bodies are further compressed, and risk of necrosis or damage shall be increased.
In the meantime, when the cutting channel is established in the blade with the same thickness as the protruding distance of the pin end, because the structured fabrics as the artificial intervertebral disc is grasped by blades with 1.0 mm to 2.0 (3.0) mm of thickness, which are thinner than the above-mentioned case, the expansion of the intervertebral space is controlled However, in this case, a situation where the structured fabrics of the artificial intervertebral disc is pressed and fitted so as to be along the endplate surface contour (shape) exposed in the reduced intervertebral space by the curettage cannot be obtained. In order to accelerate the bonding behavior at the interface between the endplates of the vertebral bodies and the structured fabrics as the artificial intervertebral disc, a slightly thicker structured fabric should be inserted into the intervertebral space and should be in the closely-attached state by press fitting. To that end, a method where the structured fabric is compressed from top and bottom with two blades having thickness, which is the same as the protruding distance of the pin end portion, for example, 1.0 mm to 2.0 (3.0) mm, and while the thickness of the structured fabric becomes thinner, it is inserted into the intervertebral space and set at the predetermined positions, and after the blades are removed, the structured fabric is compressed by the pressure of the upper and lower vertebral bodies and both end portions are fixed by the protruding pins shall be adopted. However, if this method is used, since the pin end portions shall protrude from the surfaces of the blades to allow for the thickness of these blades to compress the structure fabric by the blades, this is still the same situation where the intervertebral space has to be expanded by the entire length of the pins. In either case, even in the case of using the blades having a cutting channel where the top and bottom are penetrated through in order to minimize the expansion of the distance between vertebral bodies during operation, the blades must have physical strength to enable to compress the structured fabric as the artificial intervertebral disc by the thickness required for press fitting.
However, in actual, extremely great force is required to compress the structured fabric as artificial intervertebral disc even by 1.0 mm to 2.0 mm of thickness required for press fitting. For example, even in the case of the cervical spine, approximately 80 N (Newton), which is the average weight of human head, is required. Least of all, in the lumbar spine, several times higher compressive force is required than the cervical spine. In other words, the structured fabric as the artificial intervertebral disc for the cervical spine is designed to be deformed moderately with move ability responding to this compression force. In order to execute this press strength by two stainless steel blades at the end portion of the catalyst, greater than 3.5 mm thickness is required even in the case of a structured fabric as the artificial intervertebral disc for the cervical spine. If the blade is thinner than this, it warps outward and cannot compress the structured fabric evenly. Even if it is assumed to compress the structured fabric from the top and bottom by 0.5 mm each using the blades with 3.5 mm of thickness, it is necessary to expand the upper and lower vertebral bodies by at least 6.0 mm. In actuality, since it is necessary to expand the upper and lower vertebral bodies by 1.0 mm to 2.0 mm extra in order to smoothen the insertion, even in the case of insertion under compressing, 7.0 mm to 8.0 mm or greater has to be expanded. It is greatly possible to cause necrosis by compressing the adjacent intervertebral disc(s) only with keeping of such great intervertebral expansion for several minutes or longer, and it is extremely dangerous. Therefore, a method for press fitting the structured fabric to the endplate surfaces of the upper and lower vertebral bodies using such thick blades should not be clinically adopted.
As the protruding length of the pin end portions is longer (≧1.0 mm), stable stand-alone can be obtained with reliable fixation, but because of the reason above, the distance cannot be so long. Then, when it is desired to increase the protruding length of the pin at the end portions, a device to carve out a keeling channel with the same depth as the protruding length of the pin at the end portions in the endplates of the upper and lower vertebral bodies, and to penetrate the pin end portions through this channel for placing them in place can be considered. Therewith, it becomes unnecessary to expand a gap between the upper and lower vertebral bodies by the protruding length at the pin ends. This method is adopted to a currently ball and socket type of artificial intervertebral disc clinically in use, such as a two-layer structure of metal/polymer or a three-layer structure of metal/polymer/metal. However, with this method, since a healthy vertebral body is immoderately damaged only for the purpose of insertion and a combination of some adverse effects cannot be avoided, it should be avoided clinically.
Another issue is to suppress the original mobility of the structured fabric resulting from pins that penetrate through the structured fabric as the artificial intervertebral disc or that are intrude in the inside of the structured fabric.
In general, original dynamic behavior of the structured fabric upon compression or decompression is not comparatively controlled when the structured fabric is loaded heightwise (vertically), but because the constructed fiber in the X, Y and Z axes of the structured fabric is pulled by the rigid pins upon lateral bending and flexion/extension motions, the original deformation cannot be realized freely. The inventor of the present invention confirmed that the structured fabric buckles inward at the intermediate portion of the thickness of the structured fabric during the movement described above. In other words, in an experiment in vitro, observing motion pictures where these movements are simulated by interposing between artificial vertebral bodies, in the case of forward bending or backward bending, because the filament around the periphery of the structured fabric is tensioned by the pins and the structured fabric is constrained, the front surface and the rear surface of the structured fabric causes a notable buckling phenomenon inward from the intermediate portion. When this phenomenon is repeated for the long-term dynamic movement, because the fibers become overloaded, this causes deterioration or fracture of the fibers or structured fabric damage. Then, it is unavoidable to result in the destruction of the structured fabric for a long time after implanting. Similarly, a natural behavior of the structured fabric is inhibited at the time of torsional motion, as well. In other words, since the movable inhibitory effects of rigid pins causes the reduction in a ROM (range of motion) value, which is a value for rough indication of the mobility of the artificial intervertebral disc, a use of intrusion/penetration pins with such rigidity has to be avoided.
As another issue, because the pins easily move vertically within the structured fabric and the protruding distance of the pin end portions varies, the fitting of the vertebral bodies into the holes punched in the endplates is not certain, and it causes a lack of fixation reliability. Further, while they are loaded and the structured fabric is dynamically deformed, the pins themselves is bent due to their deformation force, and it is possible to extract the structured fabric from the holes punched in the endplate and to dislocate it from the disc space, thus a lack of fixation for a long term is also mentioned.